In recent years, with the growing awareness of global environmental conservation, the expansion of electric vehicles and plug-in hybrid vehicles is in demand. These cars include a main battery that supplies power to a motor during traveling. When charging this main battery from a commercial alternating-current power supply, there is a need for a power supply device that has a function for insulating between the commercial power supply and the main battery to charge with less power and in a safe manner. This power supply device needs to have high conversion efficiency.
An electric vehicle includes an insulated direct current to direct current (DC-DC) converter mounted therein that supplies power from a high-voltage main battery to a load of an auxiliary equipment system of an electrical component during traveling. Since the range that the voltage of the main battery fluctuates is large between in the fully charged state where the voltage is maximum and in the fully discharged state where the voltage is minimum, the insulated DC-DC converter is required to operate with high efficiency over a wide operating voltage range. However, when the range of the input voltage to the insulated DC-DC converter is wide, the control duty of a switching power supply needs to be narrowed depending on conditions, and this may reduce the conversion efficiency.
For the purpose of converting power with high efficiency, PTL 1 discloses a configuration in which a DC-DC converter and an insulated DC-DC converter are connected in series, and a conversion ratio of the insulated DC-DC converter is set to fixed magnification and the first (non-insulated) DC-DC converter to variable magnification, with the aim of improving conversion efficiency of overall converters.